1. Field of the Invention
The present invention relates to 3-dimensional (3-D) video displays and particularly to the digital formatters used in such displays.
2. Description of the Related Art
3-D display applications using a sequential left-eye, right-eye technique require a high frame rate, usually on the order of 96-120 frames/sec, to avoid objectionable flicker. This rate is accomplished in some 3-D film projection systems using two synchronized film projectors, one with left-eye information and the other with right-eye information, each running at a 24-frame/sec temporal rate, but at a 48-frame/sec display (flash) rate. A correspondingly synchronized viewing device, typically a shutter device such as a visor, helmet, or goggles is worn by the viewer. The viewer-worn shutter device shutters the left-eye and right-eye such that a 3-D image is perceived. The viewer device can also be various forms of polarized elements that allow light of a unique polarization to pass to each eye. As shown in FIG. 1, for the right-eye data frame 10 and left-eye data frame 15, a shutter is opened twice per film frame with a 50% duty cycle, allowing the two projectors to run precisely out of phase. That is, when the right-eye frame AR 11 is displayed, the corresponding left-eye frame 16 is shuttered OFF and when the left-eye frame AL 17 is displayed, the corresponding right-eye frame 12 is shuttered OFF. The display of frames AR and AL are then repeated as frames AR 13 and AL 19, respectively. Again, when right-eye frame AR 13 is displayed, the corresponding left-eye frame 18 is shuttered OFF, and when left-eye frame AL 19 is displayed, the corresponding right-eye frame 14 is shuttered OFF. This effect can also be produced by differently polarized light used for the left and right projection combined with polarized viewing equipment. Using this technique, frames are processed at a temporal rate of 24-frames/sec and displayed twice per frame to provide a flash rate of 48-frames/sec for each eye or an effective overall 3-D display flash rate of 96-frames/sec for both eyes.
Modern digital projection display systems provide flicker-free performance operating at 30 frames/sec (60-fields/sec, interlaced) rates, as illustrated in FIG. 2. Here, the frame 20 consists of interleaved Field A 21 and Field B 22, each occurring at a 60-field/sec rate. This means that each frame of 30 frame/sec data is flashed on to the screen in interleaved half fields at 60 fields/sec. Other displays, such as Digital Micromirror Device (DMD) projections displays, operate at 60 progressive (non-interlaced) frames/sec rates where every line is displayed in every frame. However, in order to avoid flicker and maintain a perceived fusion of motion, a 3-D version of such a projection display requires display rates of up to 120-frames/sec (twice the normal rate) due to the sequential left-eye, right-eye technique involved.
FIG. 3 shows the format that is typically used when displaying 24-frame/sec film (cinema type) media on a 2-D digital projection display. The 24-frame/sec film data 30 is converted to 60 field/sec video 31 using a 3:2 pull-down technique as discussed in the referenced patent application (No. TI-26774). Every other frame of 24-frame/sec film data 30 is broken into two or three 60 field/sec interlaced video data fields 31, respectively. That is, film frames A 300 and C 302 are converted to two video fields A1 310, A2 311 and C2 315, C1 316 while film frames B 301 and D 303 are converted to video fields B1 312, B2 313, B1 314, and D2 317, D1 318, D2 319, respectively. The process then repeats over and over for every four frames of film data 30.
The extra field in every other frame can present artifacts in the projected video. This can be overcome by converting the 60-field/sec interlaced video 31 to 24-frame/sec progressive (non-interlaced) video 32. This is accomplished by discarding one of each extra fields of interlaced video, B1 312 or B1 314 and D2 317 or D2 319; e.g., selecting A1 310 (frame 1, field 1) and A2 311 (frame 1, field 2) as progressive video frame A 320, B1 312 (frame 2, field 1) and B2 313 (frame 2, field 2), while discarding the data for B1 314 (frame 3, field 1), as progressive video frame B 321, C2 315 (frame 3, field 2) and C1 316 (frame 4, field 1) as progressive video frame C 322, and finally D2 317 (frame 4, field 2) and D1 318 (frame 5, field 1), while discarding the data for D2 319 (frame 5, field 2), as progressive video frame D 323. This data format, resulting in 24-frames/sec of progressive video, can be used in each eye of a 3-dimensional digital projection display to provide video that is free of 3:2 pull-down artifacts.
FIG. 4 is a block diagram showing how video data is handled in a 2-D digital projection display, such as a Digital Light Processor (DLP™) projector. The system consists of data processing circuitry 40, which takes the video input signal and performs such functions as correction for brightness, contrast, chroma interpolation, and color space conversion, a digital formatter 41, two memory buffers 42-43, and in this case three Digital Micromirror Devices (DMDs) 44-46. In operation, while video data from one of the memory buffers 42 or 43 is being presented to the three DMDs for display, the next frame of processed video data is being loaded into the other memory buffer 43 or 42, respectively. As a result, the next frame of video data is always being prepared while the present frame is being displayed. As pointed out in the discussion of FIG. 1, a digital 3-D display will require display rates of 96-120 fields/sec to avoid flicker, but because each frame of video is flashed twice per frame (repeated), the process rate of the data need only be 48-60 fields/sec.
The earlier invention disclosed and claimed in U.S. patent application Ser. No. 09/154,461 entitled “Artifact Elimination for Spatial Light Modulator Displays of Data Converted from Film Format” is relevant as background information to the current invention.
There is a recognized need for a 3-D display, which can handle the bandwidth (frame rate) requirements discussed above at an affordable price and still provide high-performance video. The invention disclosed herein addresses this need in terms of both a method and an apparatus.